Paint containing aluminum pigment coated with an electrically insulating coating



United States Patent 3,210,316 PAINT CONTAINING ALUMINUM PIGMENT COATED WITH AN ELECTRICALLY INSU- LATING COATING John W. Merck and Lester L. Spiller, Indianapolis, Ind.,

assignors to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana No Drawing. Filed July 24, 1961, Ser. No. 125,933 8 Claims. (Cl. 26039) This invention relates to coating materials for use in electrostatic spraying or coating systems, and to methods for preparing such coating materials.

In the operation of certain types of electrostatic spray coating systems employing multikilovolt potentials, it has been found that the use of paints or other liquid coating materials which are electroconductive, or which become electroconductive under operating conditions, frequently presents serious problems. One type of such system is that in which satisfactory operation involves the maintenanee of a high potential difference across the column of paint, or a portion of such column, flowing to the site of atomization. Another system in which too great conductivity of the paint creates a disadvantage is one in which paint exists in the form of an extended film on the surface of an atomizing head designed, for safety reasons, to possess a low effective electrical capacitance. If the paint forming a film on the surface of such an atomizing head is electroconductive, the effective electrical capacitance of the head and the paint film on it is greatly in creased, perhaps to a point where objectionable fire and shock hazards would exist.

One type of paint which causes problems as a result of its electroconductivity, or of its capability of becoming electroconductive under operating conditions, is one containing electroconductive pigment particles suspended in a nonconductive continuum. Paints in which the pigment contains metallic flakes, such as aluminum flakes, are of this type, particularly if the flake content of the paint exceeds about one and one half ounces per gallon. Electrical conductivity possessed by such paints under operating conditions frequently precludes their use in electrostatic spray coating apparatus of the types above mentioned.

According to this invention, conductive pigment particles to be suspended in a non-conductive continuum are given a non-conductive coating. In its broader aspects, the invention is not concerned with the type of conductive pigment or the type of insulating coating provided on the; pigment particles; but in its narrower aspects, the invention is concerned with paints containing pigment in the form of metallic flakes, particularly aluminum flakes.

For best results, the insulating coating should meet several requirements. It should be effectively continuous over the conductive particles in order to prevent electroconductive contact between particles; it should be sufliciently transparent to avoid undue reduction in the reflection of light from the surface of the substrate; it should not react with or dissolve in the vehicle in which pigment is to be suspended or, in the case of a metal-flake pigment, be of such a character that it would prevent the flakes from assuming, when the paint is applied, the imbricated arrangement necessary to the luster desired in metal-flake paint finishes; and it should be adequately adherent and sufficiently flexible that it will not chip to any undue extent.

It has been found that fluorocarbon compounds meet the above requirements fairly well, although the proportions required are relatively high; and that fact, together with the comparatively high cost of such compounds, ren- 3,210,316 Patented Oct. 5, 1965 "ice ders their commercial use impracticable at the present time.

Since aluminum oxide is an effective insulator, oxidizing the surface of aluminum flakes suggests itself as a means for providing the desired insulating coatingson aluminum flakes. It has been found, however, that oxidizing the flake surfaces to the extent necessary to provide an adequate insulating effect results in a coating so thick as to detract appreciably from the desired appearance of the finish. This may be due to the fact that aluminum oxide films on a metallic aluminum substrate are foraminous, thus providing an opportunity for electrical contact between two interengaging aluminum flakes.

By treating lightly oxidized surfaces of aluminum flakes, with a material such as sodium silicate, it is possible to decrease the conductivity of a paint in which such flakes constitute a pigment. However, it has been found that the sodium silicate treatment leaves something to be desired because of its tendency to detract from the luster desired in a finish produced by such a paint.

A preferred method for treating aluminum flake pigment to render nonconductive a paint containing it, while at the same time preserving the desired luster in the finish produced by the paint, involves forming a thin oxide coating on the surface of the aluminum flakes and then treating the flakes with a solution of sodium fluosilicate (Na SiF The evidence indicates that, as the result of such a treatment, the sodium fluosilicate reacts with the aluminum oxide to form an inorganic polymer and to create an effectively continuous, electrically nonconductive film over the surface of each flake. By properly selecting the conditions of treatment, such film may be made thick enough to provide adequate electrical insulation While at the same time being thin enough as not to interfere objectionably with the light reflective properties of the flakes.

In the preferred method of treating aluminum flakes, a commercial form of non-leafing flake-aluminum paste, such as the MD 787 grade marketed by Metals Disintegrating Co. or the Alcoa 226 grade marketed by Aluminum Company of America, is employed as a starting material. Such pastes contain approximately 65% of flake aluminum and about 35% mineral spirits, with the aluminum flakes having been treated with a small proportion of oleic or other fatty acid to control leafing properties. As a first step in the processing treatment, the paste is washed with acetone to remove fatty acid in order to condition the pigment for subsequent treatment with aqueous solutions; and after the washing, the acetone is removed, as by filtering or centrifuging. The washed pigment is then mixed with several times its weight of a 3% (by Weight) solution of sodium bicarbonate in water, and the mixture stirred vigorously for ten minutes, after which the mix is allowed to stand for minutes, with occasional stirring, for a total reaction time of minutes. This reaction, which may be carried out at room temperature, provides a thin oxide film on the aluminum flakes. If desired, a wetting agent, such as one of fluorochemical type, may be incorporated in the sodium bicarbonate solution to reduce the amount of agitation required in the production of the desired reaction.

After completion of the oxidizing reaction, the oxidized flakes are separated from the sodium bicarbonate solution, as by filtering or centrifuging, and then added to a 0.05% solution of sodium fluosilicate in water. The quantity of sodium fluosilicate solution employed is desirably about seven times the weight of the oxidized pigment. After about 15 minutes with occasional stirring, the pigment is separated from the solution, as by filtering or centrifuging, and then washed, preferably twice, with one and one-half to two times its Weight of acetone. Following the acetone wash, the acetone is removed by thorough washing with a water-free non-polar solvent such as Xylol, mineral spirits, or other hydrocarbon which may be, and conveniently is, a thinning ingredient of the vehicle in which the pigment is to be suspended.

Pigment treated as above described, when suspended in an appropriate vehicle having electrical insulating characteristics, produces a paint which is electrically nonconductive and which does not become objectionably conductive under conditions of use, even if the pigment content is as high as 24 ounces per gallon. One suitable vehicle for a pigment treated as above described consists of approximately 28% alkyd resin (100% solids), 14.5% urea-formaldehyde resin (100% solids) in xylol, 57% solvents, and 0.5% amine additive.

Many aluminum flake paints contain more or less other pigment for the purpose of adding color. It has been found that when the aluminum flakes of such a paint have been treated as above described the amount of color pigment required to give any desired effect is substantially less than that required when the flakes are untreated. The treated flakes remain in suspension as well as, or even better than untreated flakes.

The coatings formed on aluminum flakes by the process above described are fairly dense, glossy in appearance, and possess refractive indexes close to those for normal paint vehicles. If not too thick, they do not noticeably affect the appearance of the finish produced by the paint. Overly thick coatings not only tend to affect appearance of the finish, but are additionally objectionable because they tend to be brittle and subject to chipping. Objectionable brittleness is likely to exist if the coatings exceed 1000 angstrom units in thickness, and materially thinner coatings will provide adequate insulating properties. Coatings on the order of 50 angstrom units are preferred.

In order to prevent deterioration of the oxide coatings, as well as to lessen or eliminate the difficulty of suspending the treated pigment in a hydrophobic vehicle, it is desirable that the pigment be relatively moisturefree. Moisture contents of more than about 0.1% are objectionable. One purpose of the last acetone washes of the process above described is to remove moisture residues resulting from the bicarbonate and fluosilicate treatments.

It will be understood, of course, that references to liquid coating materials as conductive or non-conductive are to be interpreted in a relative sense. The resistivity which a paint must possess to be suitable in an electrostatic spraying apparatus will depend upon the nature of the apparatus. The specific alkyd-urea vehicle referred to above has a volume resistivity of about 20 megohmcentimeters and is adequately resistive to permit its use in electrostatic spraying apparatus. The addition to such a vehicle of aluminum flake pigment treated as above described works no significant change in resistivity. On the other hand, the addition of untreated aluminum flake pigment may reduce the volume resistivity to about 2 megohm centimeters, or even less; and a paint whose resistivity is of that order is frequently unsuitable for use in electrostatic spraying apparatus.

What is claimed is:

1. A paint comprising a nonconductive liquid con tinuum and, suspended in said continuum, a pigment in the form of aluminum flakes coated with an electrically nonconductive coating containing the reaction product of an aluminum oxide and a fluosilicate.

2. A paint as set forth in claim 1 wherein the fluosilicate is sodium fluosilicate.

3. A paint as set forth in claim 1 wherein the aluminum flakes are present in an amount in excess of one and one-half ounces per gallon.

4. A paint as set forth in claim 3 wherein the aluminum flakes are present in an amount up to 24 ounces per gallon.

5. A method of providing an electrically insulating coating on aluminum flakes to be used as a paint pigment, comprising oxidizing the surface of said flakes to provide a thin oxide coating on the flakes and reacting said coating with a fluosilicate.

6. A method as set forth in claim 5 wherein the oxidizing is performed by agitating an aqueous solution of sodium bicarbonate containing the flakes and the reacting is performed by treating the oxidized flakes with an aqueous solution of sodium fluosilicate.

7. A method as set forth in claim 5 wherein the oxidizing is performed by agitating an aqueous solution of sodium bicarbonate containing the flakes.

8. A method as set forth in claim 5 wherein the reacting is performed by treating the oxidized flakes with an aqueous solution of sodium fluosilicate.

References Cited by the Examiner UNITED STATES PATENTS 2,544,139 3/51 Deyrup et al 148-627 X 2,796,370 6/57 Ostrander et al 148-627 X 2,885,366 5/59 Iler 117-100 2,899,335 8/59 Straughan 117-37 2,926,106 2/60 Gauthier 239-15 X 3,016,876 1/62 Sedlacsik 117-93.44 3,026,220 3/62 Sowards 148-627 3,104,169 9/63 Metcalfe et al 117-37 3,108,893 10/63 Oliphant 117-934 WILLIAM D. MARTIN, Primary Examiner.

RICHARD l). NEVIUS, Examiner, 

1. A PAINT COMPRISING A NONCONDUCTIVE LIQUID CONTINUUM AND, SUSPENDED IN SAID CONTINUUM, A PIGMENT IN THE FORM OF ALUMINUUM FLAKES COATED WITH AN ELECTRICALLY NONCONDUCTIVE COATING CONTAINING THE REACTION PRODUCT OF AN ALUMINUM OXIDE AND A FLUOSILICATE.
 5. A METHOD OF PROVIDING AN ELECTRICALLY ENSULATING COATING ON ALUMINUM FLAKES TO BE USED AS A PAINT PIGMENT, COMPRISING OXIDIZING THE SURFACE OF SAID FLAKES TO PROVIDE A THIN OXIDE COATING ON THE FLAKES AND REACTING SAID COATING WITH A FLUOSILICATE. 